Originally posted by gazonk Actually, I think you two agree
Things aren't as simple as they appear to be.
Obviously, a smaller sensor allows for a smaller image circle. And a smaller focal length at same field of view.
And at the same f-stop, the smaller focal length allows for a smaller lens diameter, actually proportionally shrunk to the image circle.
The smaller image circle only helps decreasing a lens' size if it is larger than the lens diameter in the first place. Which is the case only for short to slow medium focal lenses. I.e., fast medium and long focal lenses don't benefit from a smaller image circle.
It is even more complicated: the smaller image circle requires smaller f-stops for similiar performance requiring higher refraction and more and thicker lens elements for a similiar performance in LW/PH. Therefore, for a given image quality, the smaller image circle can require the larger lens.